The present invention relates to a method for driving a consumer according to the preamble of claim 1 and to a control arrangement for driving a hydraulic consumer according to the preamble of claim 2.
Such a control arrangement is known, for example, from WO 95/32364 A1. In this known approach, a variable pump is controlled in such a way that it produces a pressure at its output that exceeds the highest load pressure of all hydraulic consumers of the control arrangement by a certain differential amount. To do this, constant pumps in combination with a threeway flow control valve or variable pumps having a variable stroke volume may be used.
With variable pumps, a load-sensing regulator is provided for such load-sensing controls, where the pump pressure is applicable in order to reduce the volume of the variable pump and where, in order to increase the stroke volume of the pump, the maximum load pressure and a pressure spring are applicable. The difference between the pump pressure and the maximum load pressure corresponds to the force exerted by said pressure spring. In said load-sensing circuits, each consumer has associated with it a variable measuring orifice as well as an upstream or downstream pressure compensator, through which the pressure drop across the measuring orifice is kept constant so that the amount of pressure fluid flowing to a hydraulic consumer depends solely on the opening cross section of the measuring orifice rather than on the load pressure of the consumer or on the pump pressure When the pressure compensators are downstream of the measuring orifice and when the pump has been varied to the maximum pressure volume and the pressure fluid flow is not sufficient for maintaining the predetermined pressure drop across the measuring orifices of all consumers, the pressure compensators of all of the driven hydraulic consumers are varied in the closing direction, so that all pressure fluid flows directed to the individual consumers are reduced by the same percentage. In such a load-independent flow distribution (LIFD), all driven consumers then move at a velocity reduced by the same value.
In LIFD systems, the flow channels for indicating the maximum load pressure for pump control and the pressure springs of the individual pressure compensators are designed in such a way that the load pressure is indicated to the pump regulator without falsification.
In some applications the hydraulic pump provides a stand-by pressure, for example at 20 bar (284.4 psi), which is needed for driving a number of consumers or valve arrangements. The pressure differential corresponding to the stand-by pressure must be reduced at the measuring orifices associated with the other consumers, so that considerable energy losses occur.
To alleviate this, it is an object of the invention to create a method and a control arrangement for driving at least one hydraulic consumer while keeping the energy losses at a minimum.
With reference to the method, the object is solved by the features of claim 1 and, with reference to the control arrangement, by the features of claim 2.
While in the prior art load sensing systems the control spring of the pressure compensator has always been designed as a weak spring, so as not to falsify the load pressure indicated to the hydraulic pump when the pressure compensator is completely open, according to the invention, however, a reduced load pressure is indicated to the pump. The stroke volume of the pump is adjusted as a function of said indicated (reduced) load pressure so that the pressure loss across the measuring orifice is smaller than the pressure differential at the pump regulator (variable pump). This means that the pressure drop across the measuring orifice is reduced as compared with the conventional approaches so that a corresponding energy economy is also achieved.
In the control arrangement used for carrying out the method said reduction of the load pressure indicated to the pump regulating means is achieved by appropriately designing the control spring acting on a control piston of the pressure compensator. Said spring is designed to have a considerably higher spring stiffness or bias as compared to the prior aria so that the spring force roughly corresponds to the pressure by which the load pressure indicated to the pump regulator is to be reduced compared to the load pressure actually applied. This means that the control arrangement differs from the prior art approaches essentially in the choice of the spring, so that existing control arrangements may easily be upgraded.
When using a control spring having an increased spring stiffness or increased bias, the effective spring force is preferably adjusted in such a manner that it corresponds to about half the pressure differential applied to the pump regulator or being present as a pressure drop at the prior art measuring orifice.
The response performance of the control arrangement is particularly advantageous when the spring force of the spring remains constant over the entirety of the stroke, i.e. ranging from a position where the control piston is completely closed to a completely open position. This can easily be achieved especially by a convenient pressure fluid flow control in which the flow forces resulting from the pressure fluid flow act in the closing direction as well as in the opening direction of the pressure compensator, and by choosing the flow forces in such a way that, together with the force of the control spring, they add up to a constant independent of the stroke of the control piston.
Such a pressure fluid flow control is known for example from the later publication of German Patent Application No. P 198 36 564.0, which disclosure is included herein by reference.
For the case that limiting the load pressure in the load pressure indicating line leading up to the pump is provided by a pressure limiting valve, the pressure compensator is preferably provided with a nozzle bore through which, when the pressure compensator is completely open, the load pressure is fed into the load pressure channel. When a plurality of pressure compensators are completely open and when the pressure limiting valve is open, the loss flows are reduced through the nozzle bores in the pressure compensators associated with the individual consumers. Providing such a nozzle bore is also in contrast to the designs previously used in load sensing systems, since conventionallyxe2x80x94as mentioned abovexe2x80x94always an unfalsified load pressure was indicated to the pump regulating means. For this reason, the hydraulic resistance of the flow channel extending to the pump regulator has always been chosen to be as small as possible, so that the pressure drop and a falsification of the load pressure is as small as possible when the pressure compensator is completely open.
The pressure in the load pressure indicating line is preferably indicated through a further communication bore in the pressure compensator to the spring chamber of the control piston, said communication bore comprising an damping nozzle for damping pressure variations.
The control arrangement according to the invention can be designed having a variable pump and an associated control unit or a constant pump having an input pressure compensator (three-way flow control valve).
Other advantageous developments of the invention are the subject matter of the dependent claims.